docs: dev-tools/lkmm: Fix typo of missing file extension

Commit 1e9ddbb2cd34 ("docs: Pull LKMM documentation into dev-tools book")
failed to add a file extension in lkmm/docs/herd-representation.rst for
referencing its plane-text counterpart.

Fix it.

Fixes: 1e9ddbb2cd34 ("docs: Pull LKMM documentation into dev-tools book")
Reported-by: kernel test robot <[e-mail]>
Closes: https://lore.kernel.org/oe-kbuild-all/[e-mail]/
Signed-off-by: Akira Yokosawa <[e-mail]>
Signed-off-by: Jonathan Corbet <[e-mail]>
Message-ID: <[e-mail]>

Documentation/dev-tools/lkmm/docs/herd-representation.rst

···
 herd-representation
 -------------------
 
-Literal include of ``tools/memory-model/Documentation/herd-representation``.
+Literal include of ``tools/memory-model/Documentation/herd-representation.txt``.
 
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